1. Field of the Invention
This invention relates to an indicator, and more particularly to an improved indicator such as a speedometer or a tachometer or the like, which includes an ultraviolet lamp for illuminating indicia such as scales or numerals provided on a scale plate of the indicator.
1. Description of the Prior Art
One type of indicator known in the prior art includes an ultraviolet lamp for illuminating indicia, such as scales or numerals, which contain fluorescent brightener therein and are provided on a scale plate of the indicator. In such a conventional indicator, since the indicia are illuminated by the ultraviolet light emitted from the lamp, it was possible to obtain good illumination of the indicia even when there was very little external lighting.
In the conventional indicator, the ultraviolet lamp was provided either on the upper or the lower side of a hood cover disposed on the front side of the scale plate. Therefore, the ultraviolet light from the ultraviolet lamp is always irradiated toward the indicia on the scale plate from the upper front side or the lower front side of the scale plate. As a result, a problem arose in that the indicia located near the ultraviolet lamp receive a large quantity of light while the indicia located far from the lamp receive a small quantity of light.
This means that when the ultraviolet lamp is disposed on the upper side in the hood cover, the indicia located on the lower side of the scale plate become relatively dim in comparison with indicia located on the upper side thereof. On the other hand when the ultraviolet lamp is disposed on the lower side in the cover, the indicia located on the upper side of the scale plate become relatively dim in comparison with the indicia located on the lower side of the scale plate. Therefore, when only a single ultraviolet lamp is provided, the brightness of the indicia varies in proportion with the distance to the lamp due to spectral dispersion of the ultraviolet light. Namely illumination of the indicia is not uniform in brightness, resulting in poor overall visibility for the indicator.
In order to overcome the problem mentioned above, it has been proposed that two ultraviolet lamps be disposed on both upper and lower sides in the hood cover to illuminate the scale plate from both sides in order to equalize the brightness of the indicia.
However, even though the visibility problem could be settled by the proposed indicator, another disadvantage will arise. Namely, the proposed indicator will necessitate having to provide an additional ultraviolet lamp and an accommodating space for the additional lamp in the indicator, and this will lead to increased size and manufacturing difficulty for the indicator.